1. Field of the Invention
This invention relates to a sintered body of cordierite capable of transmitting light, and a method of preparing the same. More particularly, it is concerned with an improved sintered body of light transmitting cordierite which can be used for making not only a carrier for a catalyst exposed to a high temperature, or the like, but also a light emitting tube, the window of a high temperature furnace, a reaction tube for a light concentrating oven, or the like.
2. Description of the Prior Art
For making the window of a high temperature furnace, a reaction tube for a light concentrating oven, or any similar part of an apparatus that is required to have heat resisting and light transmitting properties, it has been usual to use transparent quartz for any part exposed to a temperature not exceeding 1000.degree. C., and light transmitting alumina for any part exposed to a higher temperature. There has, however, been a limit to the temperature at which alumina can be used, unless its protection against heat is possible, since it is low in thermal shock resistance because of its relatively high coefficient of thermal expansion [8.5.times.10.sup.-6 (/.degree. C.)]. Transparent mullite having a thermal expansion coefficient of 4.0.times.10.sup.-6 (/.degree. C.) has recently been developed, and has been practically the only alternative light transmitting ceramic material that can be used in the high temperature range in which transparent quartz cannot. It has, therefore, been necessary to explore another ceramic material having a high level of heat resistance, a low coefficient of thermal expansion, and an improved light transmitting property.
Cordierite can withstand a maximum temperature of 1400.degree. C. and has a coefficient of thermal expansion which is as low as 2.0.times.10.sup.-6 (/.degree. C.). Therefore, it has been used for making not only ordinary porcelain ware, but also devices, parts or components of which high thermal shock resistance is required, e.g. a carrier (honeycomb type carrier) for a catalyst for purifying the exhaust gas of an automobile. Studies are also under way for exploring the possibility of using cordierite for applications in which its high insulating property at high temperatures may be useful.
Cordierite ceramics are, however, difficult to sinter. The liquid-phase sintering thereof involves a great deal of difficulty in the control of the temperature at which the liquid phase is formed. The pure phase, which is equal to the stoichiometric composition, is very difficult to densify, since its densifying temperature has an allowable variation of only about 10.degree. C. Moreover, the precipitation of a different phase, such as of mullite or spinel, is likely to occur.
It is generally understood that a polycrystalline sintered ceramic body is capable of transmitting light, or transparent if it satisfies the following requirements:
(1) A polycrystalline body composed of crystal grains having little optical anisotropy, or a transparent body having a high level of regularity in crystalline orientation; PA1 (2) A densely and uniformly sintered body not having any lattice defect, impurity phase, segregation, voids, etc. in the crystal grains, or grain boundaries that may cause the scattering of light, or a body having only lattice defects, etc. which are sufficiently small, as compared with the wavelength of visible light, not to cause any scattering; PA1 (3) A body having an appropriate grain size and not having so large a number of grain boundaries as to cause the scattering of light; and PA1 (4) A sintered body having a smooth surface not causing any scattering of light.
Cordierite crystals belong to the orthorhombic system (low temperature type cordierite), or the hexagonal system (high temperature type). They are optically anisotropic and fail to satisfy the requirement as stated at (1) above. It is, therefore, necessary to realize a sintered body having an improved light transmitting property by eliminating from it any of the factors causing the scattering of light as stated at (2) to (4) above. In other words, it is necessary to make a sintered body which is highly dense and homogeneous.
In order to obtain a sintered body which is highly dense and homogeneous, it has been usual to (a) prepare a fine powder which is of high purity and homogeneous, and (b) sinter it into a body having an appropriately controlled microstructure. A number of methods, such as coprecipitation, a method using metal alkoxides, and spray drying, have been employed for preparing a powder. Referring by way of example to the method using a metal alkoxide, it has been found to be capable of forming an ultrafine homogeneous powder which can be sintered at normal pressure to yield a sintered body of high density. No sintered product has, however, been found to be free of any precipitated phase of a different substance, or any voids. It has been impossible to make by sintering at normal pressure or in atmosphere sintering any product that is satisfactory in its freedom of pores. After all, it has hitherto been impossible to produce a sintered body of cordierite having a high level of light transmittance.